A Cu2ZnSn(SySe1-y)4 absorber layer is considered the leading candidate to replace a Cu(In,Ga)(S,Se)2 absorber layer in thin films solar cells, because it only contains cheap, and abundant elements. Cu2ZnSn(SySe1-y)4 solar cells have reached power conversion efficiencies of 9.7%, clearly showing the potential of the material (Todorov et al., Adv. Mater., 2010, 22, 1-4).
Cu2ZnSn(SySe1-y)4 thin films for solar cell applications can be produced by a variety of different techniques. For the production of thin films a large number of different deposition techniques are used (e.g. evaporation techniques, sputtering, E-beam, electrodeposition, spray pyrolisis, photo-chemical deposition, spin coating, iodine transport method, printing, pulsed laser deposition). A first possibility is to deposit all elements or binary compounds at elevated temperatures such that the absorber is formed in one step. A further technique foresees that all elements or binary compounds are deposited at once (at room or elevated temperature) and then heated to re-crystallize. Finally, all elements or binary compounds may be deposited sequentially, and then heated to intermix and crystallize.
In some cases the Cu2ZnSn(SySe1-y)4 semiconductor compound is spontaneously formed on a heated substrate (e.g. coevaporation, sputtering technique), in other cases the metals or binaries are first deposited near room temperature and are then further annealed in a furnace in S/Se atmosphere in order to form Cu2ZnSn(SySe1-y)4. Amongst others, Weber et al. have applied multi stage evaporation techniques (Thin Solid Films, 517 (2009) 2524-2526). They have shown that a solar cell of 1.1% efficiency can be achieved by first depositing ZnS and subsequently S, Sn, and Cu.
In state of the art (AgxCu1-x)2ZnSn(SySe1-y)4 (x, y, z=0 . . . 1) absorber layer fabrication, a precursor film containing the metals or the metals together with selenium and/or sulfur is annealed, or heat-treated in an S/Se atmosphere according to the proposal by Katagiri et al., Solar Energy Materials and Solar Cells 49 (1997) 407-414.
However, despite such an annealing/heat treatment, tin losses have been observed and reported throughout the literature: e.g. Weber et al., JOURNAL OF APPLIED PHYSICS Vol. 107, pp. 013516 (2010) have proposed using inert gas in order to reduce the tin loss. Similar observations have been made in Weber et al., Thin Solid Films Vol. 517 (2009) pp. 2524-2526Friedlmeier et al., 14th European Photovoltaic Solar Cell Conference Barcelona, Spain 1997Redinger et al., APPLIED PHYSICS LETTERS, Vol. 97, pp. 092111 (2010); Scragg, PhD Thesis, University of Bath (England) (2010); Weber, PhD thesis, Helmholtz Zentrum Berlin (Germany) (2009).
Katagiri et al. in Solar Energy Materials and Solar Cells, 49, (1997), 407-414, have proposed addressing this problem by using an S/Se atmosphere during annealing; however, whilst this proposal achieves a reduction in tin loss, the loss cannot be completely avoided.
Annealing in furnaces is typically performed in an S/Se vapor together with different gases: Ar, N2, H2, H2/N2. Annealing in N2 gas plus elemental sulfur vapor has been described by Araki et al., Thin Solid Films, 517 (2008) 1457-1460. Annealing in N2 and 5 wt % H2S gas is disclosed in Katagiri et al., Solar Energy Materials and Solar Cells, 49 (1997) 407-414. Annealing in N2 and 20 wt % H2S gas is described in Katagiri et al., Applied Physics Express, 1 (2008) 041201. Annealing in Ar and elemental S vapor and, alternatively, annealing in Ar and 5 wt % H2S gas has been suggested by Scragg et al., Thin Solid Films, 517 (2009) 2481-2484. Annealing in N2+10 wt % H2 and elemental S vapor has been disclosed in Scragg et al., Journal of Electroanalytical Chemistry, 646 (2010) 52-59. Finally, some annealing experiments have also been carried out under vacuum.
It is therefore an object of the present invention to overcome or alleviate at least some of the disadvantageous of the known methods of manufacturing (AgxCu1-x)2ZnSn(SySe1-y)4 semiconductor thin films.